1. Field of Technology
The present invention relates to an image forming apparatus, such as a copying machine and a printer, and a control program therefor, and particularly relates to an multi-beam type image forming apparatus, which has a function of writing an image of a plurality of lines in one scan onto a recording media, such as a photoreceptor, using a laser beam from a plurality of light sources, and a control program therefor.
2. Description of Related Art
An image forming apparatus that performs an image formation of one line in a main scanning direction corresponding to image data and also performs an image formation for one page by repeating the image formation for one line in the main scanning direction in a sub scanning direction is known.
As an example, in an image forming apparatus of an electrophotographic method, a laser beam modulated corresponding to image data is scanned in the main scanning direction of an image carrier, and along with this, an image is formed on the image carrier (photoreceptor drum), which rotates in the sub scanning direction, by using the above mentioned laser beam. In this case, the laser beam is modulated by the image data based on a clock signal (pixel clock) called a dot clock.
In case when the number of rotations of a polygon mirror is increased and a modulation frequency of the laser beam is increased to perform an image formation with high resolution or at high speed, an apparatus becomes large and the cost increases. Consequently, there is known an image forming apparatus, which includes a light source, such as a plurality that is two or not less than three of laser diodes (LD), and which performs an image formation for one page by repeating an image formation for a plurality of lines in the main scanning direction corresponding to the image data in the sub scanning direction using a plurality of laser beams from this plurality of light sources for the image formation at high speed or with high resolution.
Here, FIG. 3 illustrates a concrete example (1) of an image forming apparatus, which executes an image formation for eight lines at a time using eight beams of LD#1-LD#8. In case when an interval of an arrangement of LD#1-LD#8 is set a slightly smaller than a predetermined value, or in case when a distortion of an optical system causes an interval of eight beams of LD#1-LD#8 to be slightly smaller than a predetermined value when irradiated onto an image carrier, the interval of an adjoining section of an eighth exposure in the Nth scan and a first exposure in the N+1th scan ((a) of FIG. 3) becomes wider when compared with the other exposure adjoining section.
In this case, an area of a toner image becomes large, and the density is visually recognized to be high. The section where this density has become high appears at a rate of once in every eight lines. Since a spatial frequency is high, this section is visually hard to be recognized. However, in an image formation using a screen pattern, a moire is generated by an interference with the screen pattern, and there is a problem that the image quality deteriorates.
Next, FIG. 4 illustrates a concrete example (2) of the image forming apparatus, which executes the image formation for eight lines at a time using eight beams of LD#1-LD#8. In case when an interval of an arrangement of LD#1-LD#8 is set a slightly larger than a predetermined value, or in case when a distortion of an optical system causes an interval of eight beams of LD#1-LD#8 to be slightly larger than a predetermined value when irradiated onto an image carrier, the interval of an adjoining section ((b) of FIG. 4) of an eighth exposure in the Nth scan and a first exposure in the N+1th scan becomes narrower when compared with the other exposure adjoining section.
In this case, an area of a toner image becomes smaller, and the density is visually recognized as being low. The section where this density has become low appears at a rate of once in every eight lines. Since a spatial frequency is high, this section is visually hard to be recognized. However, in an image formation using the screen pattern, a moire is generated by an interference with the screen pattern, and there is a problem that the image quality deteriorates.
Further, FIG. 5 illustrates a concrete example (3) of an image forming apparatus, which executes the image formation for eight lines at a time using eight beams of LD#1-LD#8. Here, adjoining exposures (the exposure of the first line and the exposure of a second line, the exposure of the second line and the exposure of a third line, the exposure of the third line and the exposure of a fourth line, the exposure of the fourth line and the exposure of a fifth line, the exposure of the fifth line and the exposure of a sixth line, the exposure of the sixth line and the exposure of a seventh line, and the exposure of the seventh line and the exposure of a eighth line) in Nth scan are executed simultaneously (with no time difference).
On the other hand, in an adjoining section ((c) in FIG. 5) of the exposure of the eighth line in the Nth scan and the exposure of the first line in N+1th scan, the time difference of Nth scan and N+1th scan occurs at an exposure timing.
In this case, even when beam intervals are equal, an existence of the time difference at the time of recording generates reciprocity on the photoreceptor to fail. That is, a reciprocity failure occurs. As a result, even when the beam intervals are equal, the amount of adhesion of the toner differs and the density of an image changes.
In this case, a high illumination reciprocity failure is assumed to have occurred by the exposure of the laser beam. In case when the total exposure amounts are the same for a simultaneous recording of adjoining two lines and a time difference recording of adjoining two, but the exposure times differ, the sensitivity of the photoreceptor decreases as the exposure time becomes shorter. That is, when compared with the eighth line and the first line of the time difference recording ((c) in FIG. 5), the sensitivity decreases in the first line, the second to the seventh lines and the eighth line of the simultaneous recording. Further, the density of the image decreases in the other sections in FIG. 5.
In reality, a density difference in the image occurs in a state where the interval difference of FIG. 3 and FIG. 4 overlaps with the reciprocity failure of FIG. 5. With respect to the density difference generated as mentioned above, in case when the density of the adjoining two lines ((d81) in FIG. 6) of the eighth line and the next first line is high, the exposure amounts of those two lines have to be decreased. On the other hand, in case when the density of the adjoining two lines ((d81) in FIG. 6) of the eighth line and the next first line is low, the exposure amounts of those two lines have to be increased.
However, in case when this technique is used to correct the exposure amounts of the first line and the eighth line, there is a problem that the adjoining two lines of the first line and the second line ((d12) in FIG. 6) and the adjoining two lines of the seventh and the eighth lines ((d78) in FIG. 6), which fundamentally do not need to be corrected, are affected by the correction. That is, in this case, the density of the section “d81” in FIG. 6 becomes proper by the correction. However, the density of “d12” or “d78” is changed to improper by the unnecessary correction.
Consequently, in case when correcting the density of the first and the eight lines to a lower density to resolve the defect in FIG. 6, the correction of the density of the second and the seventh lines to a higher density and the correction of the density of the third and the sixth lines to a lower density are alternately performed to solve the defect of the correction being a problem in FIG. 6. FIG. 7 schematically illustrates a state of this alternating correction.
FIG. 8 illustrates a numerical value of correction of the alternating correction with a concrete example. Here, a case in which the correction for reducing the image density that has increased by performing the correction to the adjoining two lines of LD#1 and LD#8 is illustrated as the concrete example.
In this case, the corrections in the same direction adjoin at the fourth line and the fifth line (FIG. 7 (e45)) arranged in the middle. Thus, the alternating correction fails (refer to FIG. 8). That is, the same density difference that had been generated at the eighth line and the first line before the correction appears at the fourth line and the fifth line. Consequently, the density difference becomes difficult to be resolved.
The later mentioned Unexamined Japanese Patent Application Publication No. H8-76039 discloses a technique of suppressing the image quality deterioration, which is caused by an error of a beam pitch in the sub scanning direction just as described above. In the later mentioned Unexamined Japanese Patent Application Publication No. H8-76039, a countermeasure is taken so that the intervals of the plurality of laser beams are equal. For example, a technique of canceling the density difference just as described above by an adjustment of the exposure amount is disclosed in U.S. Pat. No. 2,685,345 mentioned later.
With respect to the technique of the above mentioned Unexamined Japanese Patent Application Publication No. H8-76039, there is a problem that the mechanical adjustment, such as an adjustment of an optical system, is needed. In this case, an arrangement of mechanical adjustment mechanism creates a new problem of reducing the stability and of generating a distortion.
FIGS. 9a, 9b and 9c illustrate a concrete example of the adjustment of this optical system. Here, a case in which a LD array of eight beams is used as a multi-beam is considered. Here, the exposure of the multi-beam is performed by inclining this 8-beam array by a predetermined angle θ as illustrated in FIG. 9a and setting this 8-beam array to a desired pitch p between beams (sub scanning pitch).
Here, an optical characteristic is assumed to be LD emitting point interval: 30 μm, collimator lens focal distance f_col: 30 mm, cylindrical lens focal distance f_cy: 112.8 mm and scan optical system sub scanning rate m: 1.2 times as illustrated in FIG. 9b. 
A sub scanning pitch p on the photoreceptor drum surface becomes p=7*d*sin θ*f_cy/(f_col*m).
In case when the predetermined angle θ is 9.0, an error Δp of a distortion of the angle θ and the sub scanning pitch p becomes as shown in FIG. 9c. Here, even when θ shifts from 9.0 degree to only ±0.3 degrees, a pitch error becomes approximately 5 μm (approximately ¼ pixels) that is clearly noticeable. Therefore, with respect to the technique disclosed in Unexamined Japanese Patent Application Publication No. H8-76039, a problem of stability reduction and distortion generation occurs.
In the technique of the above mentioned Japanese Patent No. 2685345, since the light volume is adjusted, a problem of light volume change in the other line as described above occurs. Also there is a problem that a negative effect of the correction cannot be completely solved.
The present invention solves the above mentioned problem. An object of the present invention is to realize an image forming apparatus and a control program therefor that is capable of properly resolving an image density difference generated by a sub scanning direction beam interval difference and a reciprocity failure at the time of an image formation with a simultaneous exposure of a plurality of lines.